As illustrated in FIG. 5, a welding start circuit ST is arranged outside of a welding power source PS. When receiving a welding start signal St from the welding start circuit ST, the welding power source PS outputs a welding voltage Vw and a welding current Iw to generate an arc and a feed control signal Fc to control the feed of a welding wire 1. As the welding start circuit ST, a program logic controller (PLC), which controls welding steps, and a robot controller may be employed. A feed roll 5 is connected to a wire feed motor WM. The welding wire 1 is fed to a base material 2 via the interior of a welding torch 4 through rotation of the feed roll 5. An arc 3 is generated between the welding wire 1 and the base material 2 in a shielded state by shield gas 6. When the wire feed motor WM rotates in a forward direction, the welding wire 1 is sent in a direction toward the base material 2 and is thus advanced. In contrast, when the wire feed motor WM rotates in a reverse direction, the welding wire 1 is moved in a direction separating from the base material 2 and thus retracted. A welding torch 4 is mounted to a robot body (not shown), an automatic carriage (not shown) or other devices. The welding torch 4 is moved in a three dimensional direction to perform welding.
When the welding wire 1 and the base material 2 are held in contact (short-circuited) or an arc is generated, the welding current Iw flows between the welding wire 1 and the base material 2. Contrastingly, when the welding wire 1 and the base material 2 are separate from each other and the current state is a no-load state in which the arc 3 is not generated, the welding voltage Vw becomes a maximum value (a no-load voltage) and the welding current Iw does not flow between the welding wire 1 and the base material 2. The distance between the distal end of the welding wire 1 and the base material 2 is a wire distal end/base material distance Lw [mm]. Accordingly, the wire distal end/base material distance Lw is substantially equal to the arc length when an arc is generated. In the following explanation, physical contact of the distal end of the welding wire 1 with the base material 2 will be referred to as contact, and electrical connection between the distal end of the welding wire 1 and the base material 2 will be referred to as short circuit. Therefore, in a contact state, the distal end of the wire contacts the base material 2 with being short-circuited or without being short-circuited. If an insulating material (such as slug) is adhered to the distal end of the welding wire 1, the distal end of the welding wire 1 and the base material 2 are not short-circuited even if they contacted each other. If insulating material has been removed from the distal end of the welding wire 1, the distal end of the welding wire 1 and the base material 2 are short-circuited if they contact each other.
FIG. 6 includes timing charts representing a conventional retract arc start control method performed by the welding apparatus illustrated in FIG. 5. FIG. 6(A) represents the welding start signal St, and FIG. 6(B) represents the feed control signal Fc. FIG. 6(C) represents the welding voltage Vw, and FIG. 6(D) represents the welding current Iw. FIG. 6(E) represents the wire distal end/base material distance Lw. The retract arc start control method will now be described with reference to FIGS. 6(A) to 6(E).
(1) Wire Slow-Down Period from Time Point t1 to Time Point t2
At time point t1, with reference to FIG. 6(A), the welding start signal St is input and reaches a High level. Then, as represented by FIG. 6(B), the feed control signal Fc becomes a slow-down feed speed Fir and the welding wire starts to be advanced. Normally, the slow-down feed speed Fir is set to a slow speed of approximately 1 to 2 m/min. This is because if the slow-down feed speed is raised, physical contact between the welding wire and the base material causes the welding wire to be strongly pressed against the base material and deformed or lifts up the welding torch 4, so that the arc start performance is deteriorated. Simultaneously, output of the welding power source PS is started and, with reference to FIG. 6(C), the welding voltage Vw is applied. Since the state at time point t1 is the non-load state, the welding voltage Vw is set to a non-load voltage Vn1, which is a maximum output voltage value (approximately 70 to 100V). After time point t1, the welding wire is advanced and the wire distal end/base material distance Lw gradually decreases as represented by FIG. 6(E).
(2) Contact Period from Time Point t2 to Time Point t3
When the distal end of the welding wire contacts the base material and is short-circuited at time point t2, the wire distal end/base material distance Lw becomes zero as represented by FIG. 6(E), and the welding voltage Vw becomes a short circuit voltage of approximately several volts as represented by FIG. 6(C). Further, the welding current Iw becomes an initial current setting value Iir (approximately 10 A to 100 A) with reference to FIG. 6(D). At this stage, by detecting the fact that the welding voltage Vw has become smaller than a predetermined reference voltage reference value Vth as represented by FIG. 6(C), it is determined that the welding wire and the base material are short-circuited. Further, at this stage, with reference to FIG. 6(B), the feed control signal Fc becomes a retract feed speed setting value Fbr having a negative value, and thus the welding wire starts to be retracted. However, in the short circuit period from time point t2 to time point t3, as represented by FIG. 6(E), the distal end of the welding wire and the base material are maintained in contact with each other due to a delay time caused by reversal of the rotation of the wire feed motor from the forward direction to the reverse direction or a delay time necessary for retracting the welding wire by a length corresponding to the play of the welding wire in the welding torch. Although the short circuit period varies depending on the type of the wire feed motor and the length of the welding torch, the short circuit period is normally 10 to 100 ms.
(3) Initial Arc Lift Period Td from Time Point t3 to Time Point t4
When the distal end of the welding wire is separated from the base material as represented by FIG. 6(E), a current corresponding to the initial current setting value Iir is supplied and an initial arc is generated. When the initial arc is produced, with reference to FIG. 6(C), the welding voltage Vw reaches an arc voltage of several tens of volts, which is higher than or equal to the reference voltage Vth. In the predetermined initial arc lift period Td (from time point t3 to time point t4), the welding wire is retracted continuously as represented by FIG. 6(B). This is because, if movement of the welding wire is switched from retract to advance immediately after the initial arc has been produced, the wire and the base material may be caused to re-contact with each other due to an insufficient arc length. In order to prevent such re-contact and smoothly switch to a steady arc state, the welding wire is continuously retracted to increase the arc length with the initial arc maintained in the initial arc lift period. The welding wire continues to be retracted until the arc length becomes substantially equal to a steady arc length. The initial current for the initial arc is maintained at the low level in order to prevent the initial arc from melting the distal end of the welding wire and causing the arc to flare up. If the arc flares up when the welding wire is retracted, it is difficult to raise the arc accurately to a desirable value. The current value of the initial current is suppressed to be low to prevent the arc from flaring up. To prevent the re-short circuit, a re-short circuit preventing current that is a short time pulse current may be supplied when the initial arc is caused at time point t3. According to this method, the pulse current of a high current value is supplied to instantly elongate the arc and prevent re-short circuit.
(4) Steady Arc State Period After Time Point t4
When the initial arc lift period Td ends at time point t4, the feed control signal Fc becomes a steady feed speed setting value Fcr as represented by FIG. 6(B) and the welding wire starts to be advanced again. Simultaneously, with reference to FIG. 6(C), the welding voltage Vw is subjected to constant voltage control to become equal to a predetermined voltage set value Vr and, as represented by FIG. 6(D), a steady welding current Ic corresponding to the steady feed speed is supplied. In this manner, with reference to FIG. 6(E), the initial arc generating state is smoothly switched to the steady arc state. In the steady arc state, the arc represents a steady arc length Lc.
In the above-described control method, constant current control is performed on the initial current by the welding power source PS so as to control the current accurately. As represented by FIG. 6(D), the initial current is constant. However, there may be cases in which the current is suppressed to a small value when the welding wire contacts the base material at time point t2 and then increased in the short circuit period. This prevents an arc from being generated, and melting and joining the welding wire and the base material together when the welding wire and the base material are in contact. In this case, the welding wire is advanced or is retracted to move the distal end of the welding wire forward or backward. However, the welding torch may be advanced or retracted to move the distal end of the welding wire forward or backward. Therefore, the distal end of the welding wire is moved forward or backward by moving the welding wire forward or backward or by moving the welding torch forward or backward. Of course, after time point t4, the welding wire is fed to move the welding wire forward again. The above-described conventional art is disclosed in, for example, Patent Documents 1, 2.
In the control method described in FIG. 6, the distal end of the welding wire physically contacts the base material at time point t2, and is simultaneously short-circuited. At this time, the forward movement of the welding wire is suspended immediately and the movement of the welding wire is switched to retracting. Therefore, the welding wire is not strongly pressed against the base material. However, in some cases, an insulating material is adhered to the distal end of the welding wire by the influence of the previous welding. Because of the insulating material, the welding wire and the base material may not be short-circuited even if the distal end of the welding wire and the base material physically contact each other. In such a case, the welding wire is continuously advanced at the slow-down feed speed to press the distal end against the base material. If the insulating material adhered to the distal end of the welding wire is destroyed by the pressing force of the welding wire, the distal end of the welding wire and the base material contact each other and are short-circuited. At this time, if the initial current is supplied to the welding wire and the welding wire is retracted to generate the initial arc in a state where the welding wire is strongly pressed against the base material, following problems are caused.
(1) Since the welding wire flexed on the feed path between the base material and the feed roll is suddenly moved toward the base material, the short circuit is caused again.
(2) Since the welding torch that has been lifted by the reaction force of the pressing force of the welding wire is suddenly moved toward its original position, the short circuit is caused again.
If the re-short circuit is caused at the time of generation of initial arc, the distal end of the welding wire is melted by the initial arc. Therefore, the distal end of the welding wire is melted and joined to the base material together. Once the welding wire and the base material are melted and joined together, a high current of several hundreds of amperes needs to be supplied to cancel the state of the melting and joining. In this case, arc start that generates many spatters is caused. This phenomenon is particularly remarkable in case of stainless steel. In the following explanation, such a phenomenon is referred to as re-short circuit caused by cancelling pressing force of the welding wire.
As described above, even if the re-short circuit preventing current is supplied at the time of generation of the initial arc, the flexed welding wire is suddenly moved toward the material to be welded, or the lifted torch suddenly moves toward its original position at a speed higher than the welding wire melting speed by the re-short circuit preventing current. This causes re-short circuit. Accordingly, in the prior art, it is difficult to prevent re-short circuit due to the removal of the pressing force of the welding wire.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-231414    Patent Document 2: Japanese Laid-Open Patent Publication No. 2007-30018